tions of this authority upon the diatoms of the Elbe, to those of Mr. T. F. Bergin on the deposits of the mud of the Liffey,* to those of Professor Bailey on the diatoms found in the Mississippi, to the paper of Mr. F. C. S. Roper on the Diatomacea of the Thames, and lastly to the memoir of Dr. Bossey on Thames Mud in relation to Sanitary Science.' Mr. Roper in 1854 and Dr. Bossey more recently have carefully studied the species of diatoms in different parts of the river, and have shown that the valves belonging to fresh-water species growing in the upper parts of the river may be carried down by the tide towards the mouth of the Thames, while the valves of those living in salt or in brackish water are to be traced as far up as the tide extends. These beautiful silicious skeletons so easily recognized and identified, being very light, are carried backwards and forwards by the tide, and are deposited on the mud-banks. They may be regarded as evidence of the course taken by other light particles suspended in the water of the river, and afford one of many indications of the movements of the sewage. Thus we are able to show that at any rate the least dense of the constituents of sewage may be carried from the outfall at Barking up to the first lock in one direction and below Gravesend in the other.

It is impossible to exaggerate the importance of investigations concerning the course of the sewage in the river considered in connection with the changes effected in it by various agencies during its suspension and after its subsidence as mud.

That our river is

fouled by the presence of sewage is patent to every one, while most of us feel that its state is a disgrace to our city. The serious question which presents itself to Londoners, and indeed concerns England, is whether this constant pollution of the river by the pouring into it daily of more than 100 millions of gallons, nearly 450,000 tons, of sewage can be continued without increasing risk to the health of the people, to say nothing of the disagreeable effects on the senses of sight and smell, and the very unpleasant considerations suggested by the contamination.

Some years ago there was unmistakable evidence of the occurrence of a very nasty kind of decomposition proceeding in the Thames water. The air of all the streets bordering the river was polluted with offensive odours. During the last few years, however, we have not been so seriously annoyed. But it must be borne in mind that we have had a remarkable series of cool and wet summers, favourable to excessive dilution of the sewage and unfavourable to organic decomposition. What the state of things

*Cooper and Busk's Microscopic Journal,' ii. (1842) p. 68. †Trans. Micr. Soc. Lond., ii. (1854) p. 67.

Proceedings of the Holmesdale Natural History Club,' December 12th,

1879.

would be if we had a very dry hot summer succeeding to a spring with less than the usual rainfall it is not pleasant to contemplate, for I am afraid it is probable that the considerable reduction of the volume of water in proportion to the sewage would result in a concentration of the dissolved and suspended organic matters, which, gradually rising in temperature from day to day to 70° or higher, would perhaps almost suddenly undergo a form of putrefactive change resulting in the setting free of large volumes of highly fetid gases, which would poison the air far and wide. Such a nuisance might persist for weeks, and only disappear when by the autumn rains the tidal water had become greatly diluted and its volume increased by fresh water pouring in from above. How far such a state of the river would be injurious to health it is not possible to say. I do not think anything of the kind upon so large a scale has ever happened, and any suggestion as to possible danger to health, not being backed by actual facts, would only excite counter observations and assertions as to the excellent health enjoyed by those who spend much of their time in the sewers, and a review of facts, carefully selected by no impartial hand, with the object of convincing people that stinks were not unwholesome, and that possibly to the trained they might be actually enjoyable; that the presence of decomposing animal and vegetable matter suspended in water was rather an advantage than otherwise; that countless multitudes of harmless organisms while ministering to their own enjoyment and advantage, exerted a beneficent influence by appropriating the products of disintegration just prior to decomposition; and that upon the whole we ought to consider ourselves fortunate in possessing in our midst a large river reeking with filth, because in this way the noxious substances are slowly resolved into simpler gaseous and soluble matters instead of the whole contributing to increase the already sufficiently ample mudbanks, which—and at a constantly accelerating rate-would add to the difficulties of navigation, and at length interfere with the passage of all but the smallest craft.

Method of Examination.

The large amount of gritty silicious particles, as well as their considerable size, renders the examination of small portions of mud just as it is obtained from the mud-bank very difficult. The layer placed on the glass slide and covered with thin glass will be too thick for examination by any but the lowest powers, and in consequence, some of the most minute but most important of the constituents of the mud will not be discerned. If a little of any

specimen of mud be mixed with water, covered with thin glass, and then examined in the usual way, nothing but large sand-grains,

with here and there black particles of coal or carbon, will be seen. By mixing a small portion of mud with a considerable quantity of water, stirring it up, and then pouring off the upper part of the fluid after allowing a few seconds for the subsidence of the heaviest and coarsest particles, a deposit may be obtained in a state fit for examination under tolerably high magnifying powers, and if the process be repeated again and again, the mud may be separated into several portions differing from one another in density and in the coarseness of the gritty particles. But this plan is found not altogether satisfactory, for many of the organic substances in the mud are only imperfectly seen, while it will be impossible for the observer to form any idea of the relative proportions of the various constituents of the mud thus divided into separate portions differing from one another as regards the size and lightness of the component particles.

After having tried many different methods of investigation, I found that admixture with an equal quantity of glycerine afforded the best results. In this process the specimen can be kept for a length of time without undergoing change and be submitted to examination at intervals. The refracting property of the glycerine enables the observer to make out details of structure which could not be seen in specimens immersed in water, while in each specimen almost all the constituents of the mud are rendered clearer and more distinct.

Another important advantage is gained by this method of examination, inasmuch as the observer is able to form a notion of the relative amount of the several substances in each specimen examined, and also the relative amount of each in any given specimen. By this plan every constituent of the mud may be seen in one preparation, and specimens prepared in this manner have the additional advantage of preserving their characters for many years without change.

If a portion of the mud is simply mixed with water and then stirred up, the heavier particles allowed to settle while the lighter ones are poured off into another vessel and then allowed to subside, a very wrong idea may be formed of the number of the lighter substances present, because nearly the whole of these in the quantity of mud operated upon may be separated, and the microscopical specimen would in that case appear as if it consisted almost entirely of this one class of constituent particles.

This paper is based upon the results obtained by the microscopical examination of twenty-five specimens of mud from various banks between Gravesend and Chelsea taken under the direction of Dr. Collingridge, the Port of London Sanitary Officer, in the course of an inquiry undertaken at the request of the City of London for the purpose of obtaining evidence to bring before the Royal

Commission appointed to consider the question of Thames Pollution. I have received permission to communicate the results to the Society, and to publish them.

The observations made by me relate chiefly to the organized constituents of the sewage which can be demonstrated in the mud of the Thames by microscopical examination. Many of the particles found in the mud have been identified as substances which had entered into the formation of human excrements. I have endeavoured to ascertain what changes some of the most important of the fæcal constituents undergo in their passage from the houses along the drains into the river until their disintegration is at last completed or they have been deposited and form part of the mud banks of the Thames.

The broad and important fact which is, in my judgment, fully established by the investigation is this-that several constituents of human fæces are present in all the specimens of mud submitted to examination. The amount of these differs considerably, though no adequate means have been discovered of making an accurate estimate of the quantity of any one of them, or of instituting more than a very rough comparison between the muds obtained from different banks.

It must be borne in mind that the river mud is continually undergoing change in its character, the surface of the bank being often washed away, and old matters being mixed up with the elements of recent sewage; these being deposited together in other and perhaps distant banks, as determined by the varying quantity of water, the rate of its flow, and a number of other circumstances. Thus the mud of any given bank will vary considerably in its characters at different periods of the year, and it is quite supposable that a bank, which at one time would be found to consist of nearly pure sand, at another might seem to be almost entirely composed, at least on its surface, of the blackest and foulest organic matter undergoing rapid putrefactive changes.

It is well known that the quantity of organic matter in the mud is small. If a certain portion of the mud be dried and then exposed to a red heat for a time, the loss in bulk owing to the total destruction of the organic matter and the dissipation of all volatile substances is very slight. On the other hand it is to be remarked that neither the disagreeableness nor the danger to health of organic matter in a state of decomposition is dependent upon or varies according to the amount present. From a quantity of certain forms of organic matter so small that it would fail to turn the most delicate balance, as for example a fraction from the specimen of sewage taken from an outfall near Trinity Ballast Office, an odour of a most detestable character might emanate and be diffused over a considerable area. But it must be borne in mind that as regards